The present invention relates to improved position control devices and methods for machine tools, in which a servomotor is optimally driven and controlled in accordance with full closed loop control.
Conventionally, a position control device for a machine tool is known. The machine tool includes a servomotor serving as a driving member and, for example, a sliding shaft serving as a driven member, which is driven by the driving member. More specifically, driving of the sliding shaft is controlled in accordance with servo control, including full closed loop control and semi-closed loop control. In the control loop of the full closed loop control, feedback control is performed on data representing the position of the driven member of the machine tool, such as a table, detected by a position detecting device such as an optical scale.
In the control loop of the semi-closed loop control, feedback control is performed on the position data obtained by a position detecting device such as a rotary encoder incorporated in the servomotor. That is, the semi-closed loop control directly controls the servomotor, such that control is facilitated. However, the semi-looped control may eventually cause a steady-state deviation. In contrast, the full closed loop control directly detects and controls the position of the driven member. This eliminates the possibility of steady-state deviation, thus improving positioning accuracy advantageously.
However, since the full closed loop control directly involves the machine tool, it is necessary to set a position loop gain as a relatively low value, as compared to the semi-closed control, if the rigidity of the machine tool is relatively low. If this is the case, the response of the driven member to the controlling is hampered, leading to relatively inferior dynamic performance of the driven member including contouring accuracy.